An electrophotographic process comprises forming an electrostatic latent image on a photoreceptor made of a photoconductive substance, developing the latent image with a toner by, for example, magnetic brush development, transferring the toner image on the photoreceptor to a transfer material, such as paper or a plastic film, and fixing the transferred toner image under heat or pressure or with a solvent to provide a permanent image.
For fixing a toner image, heat-fusing methods have been mostly used. The heat-fusing methods are divided into a contact system and a non-contact system. In recent years, a contact system using a heated roll has been widely used in copying or printing equipment for business use because of its high thermal efficiency and applicability to high-speed fixing.
However, the conventional heated roll fixing method tends to involve adhesion of a toner to a heated roll, which causes stains on the next copy, called an offset phenomenon. Offset is very likely to occur particularly when a quantity of heat applied per unit time is increased to cope with an increase in fixing speed in high-speed copying.
In the heated roll fixing system, a peeling claw is provided in the fixing roll part so as to prevent a transfer material, which is generally paper, from being wound around the roll after its passage. However, the increased stress imposed to the fixing part due to the recent speeding up of copying equipment sometimes causes such troubles as release failure or image missing at the front end of the copy due to scratches by the peeling claw.
Furthermore, where a photocopied image is used as an original from which another photocopy is taken, the original copy is rubbed with a paper-feed roller of an automatic paper-feed system and, as a result, the image of the original might be blurred or stained. In the case of double-side copying or multicolor copying, when a first toner image is fixed and then subjected to a second copying operation, the first toner image is rubbed with a paper-feed roller, also getting blurred or stained. Furthermore, where a plurality of copies as originals from which a copy is to be taken are set in a copying machine and automatically fed one by one, the back side of a copy rubs the image surface of the underlying copy to cause stains or blurs on the both, resulting in reduction of image quality.
On the other hand, binder resins essential to a toner for developing electrostatic latent images include styrene polymers, e.g., polystyrene, styrene-acrylate copolymer resins, polyester resins, epoxy resins, ketone resins, maleic acid resins, phenolic resins, terpene resins, polyvinyl butyral, and polybutyl methacrylate. Binder resins for toners are particularly required to have no adverse influence on chargeability, low hygroscopicity, good compatibility with carbon black used as a colorant, good grindability, and a moderately low softening point (around 100.degree. C.) for satisfactory fixing properties. Even in using a binder resin satisfying these requirements, the above-mentioned problems cannot always be eliminated. It has therefore been demanded to develop an electrophotographic toner which has excellent release properties in heated roll fixing, causing no offset nor scratches by a peeling claw.
In order to overcome the above problems, it was proposed to add to a toner low-molecular weight polypropylene or polyethylene as a lubricant as disclosed in JP-B-52-3304, JP-B-52-3305, JP-B-57-52574, JP-B-58-58664, JP-A-58-59455, and JP-A-60-151650 (the term "JP-B" as used herein means an "examined published Japanese patent application", and the term "JP-A" as used herein means an "unexamined published Japanese patent application).
Use of the above-described lubricant produces some effects in improving release properties (i.e., anti-offset properties), preventing scratches by a peeling claw, and improving rub-off resistance of a fixed image but is not sufficient. Beside, the above-described polyolefin lubricant tends to form large domains in a toner due to its poor compatibility with the binder resin, resulting in serious deterioration in powder fluidity and cohesiveness of the toner.
It is known that the problem arising from the poor compatibility of the polyolefin lubricants with the toner binder resin can be solved by using a polyolefin-grafted polymer (see JP-A-60-457, JP-A-60-93456, JP-A-60-93457) or a modified polyolefin (see JP-A-58-63947, JP-A-59-177570, JP-A-60-3644, JP-A-62-148508, JP-A-63-191817). These methods are effective to improve dispersibility of a polyolefin lubricant thereby to prevent deterioration of the powder fluidity and cohesiveness of the toner to some extent but, in turn, impair the effect of improving release properties as essentially aimed at.